The Horne and Ostberg Morningness/Eveningness Questionnaire (MEQ) is widely used to differentiate between morning and evening types, but there is very little epidemiological evidence about the distribution of MEQ chronotypes in the general population. The purpose of the present study was to simultaneously investigate the influence of demographic, socioeconomic, and work factors on the distribution of morningness/eveningness. A New Zealand version of the MEQ was mailed to 5000 New Zealand adults, ages 30 to 49 years, who were randomly selected from the electoral rolls (55.7% response rate). A total of 2526 questionnaires were included in the analyses. According to the Horne and Ostberg classification, 49.8% of the total population was classified as morning type compared to 5.6% having an evening-type preference. However, using new cutoffs for middle-aged working adults described by Taillard et al. (2004), 24.7% of the population was morning type and 26.4% was evening type. After controlling for ethnicity, gender, and socioeconomic deprivation, participants ages 30 to 34 years were more likely to be definitely evening type (odds ratio [OR] = 1.59, p < 0.05) and less likely to be morning type (moderately morning type, OR = 0.59, p < 0.01, or definitely morning type, OR = 0.59, p < 0.05) compared to those ages 45 to 49 years. Work schedules were also important predictors of chronotype, with night workers more likely to be definitely evening type (OR = 1.49, p = 0.05) and the unemployed less likely to be moderately morning type (OR = 0.64, p < 0.05) compared to other workers. Evening types were 2.5 times more likely to report that their general health was only poor or fair compared to morning types (p < 0.01). This study confirms that the original criteria of Horne and Ostberg (1976) are not useful for classifying chronotypes in a middle-aged population. The authors conclude that morningness/eveningness preference is largely independent of ethnicity, gender, and socioeconomic position, indicating that it is a stable characteristic that may be better explained by endogenous factors.